To purify the proteins of RNA tumor viruses. To continue development of sensitive and specific assays for viral proteins. To determine the functions of virion proteins, especially in regard to their possible role in the malignant transformation of virus-infected cells. To biochemically characterize temperature sensitive mutants of murine leukemia virus in order to understand the mechanism of replication of mammalian type C viruses. To determine the involvement of reverse transcriptase containing viruses in cancers of primates.